Quote Originally Posted by ecat View Post
Digressing just a little.
Moslty fabrication costs/equipment.
Alu profile just needs cut, bolted together with suitable brackets, and with a bit care you have a pretty accurate machine.
Steel needs cut (more expensive/time consuming to do accurately than cutting alu), joined, be that via welding or drilling and brackets, which both take longer than doing similar with alu profile. If welded, you then have to deal with distortion.
Plus alu profile is also more accurate out the box so to speak, as extrusions are usually accurate within a thou, unlike steel which can vary quite a bit.

I've used alu profile for frames before, and cost wise it is more expensive, but takes less time to assemble, and for what I used it for, resulted in a lighter stiffer frame than using steel. Plus it looks cool ;)